Ok, it would take me quite a long time to explain the basics of single
crystal x-ray diffraction, but i'll try to give a short overview.

X-Ray diffraction is a technique to obtain the structure of a molecule in a
single crystal and it's packing. Every atom in a molecule is swinging
because of it's own energy. The whole molecule is dithering a thousend

dithering and thermal movement of the atoms in the molecule is reduced and
then you can try to obtain it's real structure. Every molecule likes to
have a conformation in the lowes energy state, like we. We like to sit
instead of jumping around all the time. So lets come to the technique. You
irradiate a little crystal (best is a perfect grown crystal without any
disordering, wich means perfect packing and stacking of the molecules in
the crystal) with a x-ray and the ray is diffracted because it interacts
with the electrons of every atom in every molecule. But due to a perfect
packing of the molecules and interferrences during the diffraction you
obtain a special pattern of scattered or diffracted x-rays which can be
detected by a simple film (like a photograph) or much better with a
ccd-detector. Then You make a fourier synthesis to try to obtain the phase
information, because every ray has it's own phase but this information is
lost during the measurement. If the computer finds a solution for this
problem You are on a good way to find out the stucture of a molecule.
Because if the computer solved the phase problem, then You obtain a first
model of the molecule and then You refine Your model against the measured
dataset and if the abberation becomes lesser Your model is getting better
and better. Good abberations are around 5% or less. Well, i think it makes
no sense to continue but for interested people there is a good link :

http://shelx.uni-ac.gwdg.de/xtal/xtal.htm

And yes, the thermal ellipsoids can be replaced by simple spheres or sticks
or spheres representing the van der Waals radii.

So far, bye, Shu.

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